The long-term objective of this project is to increase our knowledge of the spatial characteristics and consequences of the inhomogeneously distributed damage produced by ionizing radiation in DNA model systems such as biologically relevant centromeric and telomeric repeats, as well as in chromatin and in the DNA of living cells. This information is needed in order to develop a better understanding of the mechanisms involved in the production of DNA damage which may not be repaired, or may be misrepaired, in cells and thus may lead to deleterious health effects. Models incorporating the data from this project will be more accurate in their prediction of risks for such adverse health effects as cancer from exposure to low doses of radiation. The aims of the project are to: 1) obtain measures of the dimensions of clusters of damaged molecules and the average number of free radicals/cluster produced in DNA by radiation and to determine how these dimensions vary with the energy and type of radiation passing through the DNA as well as with the DNA base sequence, packing and hydration; 2) determine the mechanisms by which the trapped free radicals react to form damaged constituents of DNA; and 3) identify and quantitate the various structurally damaged DNA constituents and determine how the quantities of these vary as in aim 1. The approach for obtaining measures of cluster dimensions and number of trapped radicals is to use pulsed electron paramagnetic resonance spectroscopy (EPR) methodologies to study the magnetic interactions of trapped free radicals in solid DNA irradiated at 77K, from which spatial information can be derived. The damaged DNA base and sugar constituents formed as the DNA is warmed will be identified and quantitated by High Pressure Liquid Chromatography (HPLC)/Mass Spectroscopy and Gas Chromatography/ Mass Spectroscopy (GC/MS). Samples of native and synthetic DNA will also be studied at room temperature as solids and in aqueous solutions. The ratios of yields of these products will be examined to seek relationships between the spatial properties of the damaged clusters and the quantities and types of damaged products observed.